C m13/F#
The C m13 chord consists of the C (Perfect unison), D# (Minor third), G (Perfect fifth), A# (Minor seventh), D (Major ninth) and A (Major thirteenth) notes, with F# as the bass note.

How to play the C m13/F# chord
- Play F# as the lowest bass note in the left hand
- Find C (Root) on the piano keyboard
- Add D# (Minor third) — 3 semitones above C
- Add G (Perfect fifth) — 7 semitones above C
- Add A# (Minor seventh) — 10 semitones above C
- Add D (Major ninth) — 14 semitones above C
- Add A (Major thirteenth) — 21 semitones above C
- Press all 7 notes simultaneously to sound the C m13/F# chord